Preparation of [acmi] HSO4 and using as catalyst for esterification

Abstract

New Bronsted acidic ionic liquid, 1-carboxymethyl-3-methylimidazolium hydrogen sulfate [AcMI]HSO₄, has two acidic sites -COOH and HSO₄^-. It has been synthesized by three steps. First, 1-carboxymethyl-3-methylimidazolium chloride [AcMI]Cl was prepared by alkylation of 1-methylimidazole with chloroacetic acid (molar ratio is 1.5:1) under microwave irradiation in 6 min (84 % isolated yield). Then, zwitter ion 1-carboxylatmethyl-3-methylimidazolium was obtained by using Ag₂O to remove ion chloride Cl^- from [AcMI]Cl. At last, concentrated sulfuric acid (98 %) was added into zwitter ion to give 1-carboxymethyl-3-methylimidazolium hydrogen sulfate (yield 96 %). This ionic liquid used as a recyclabe catalyst for the esterification of isopropanol and chloroacetic acid. The optimal reaction conditions were obtained as follows: isopropanol: chloroacetic acid:[AcMI]HSO₄ are 1.3:1:0.2, reaction time for 10 min at 60 ^oC under microvave irradiation. The yield of isopropyl chloroacetate was 86 %. This ionic liquid was removed from ester easily, recovered and recycled without loss of activity.